The present invention relates generally to the electrical stimulation of bone growth and specifically to an improved cathode which can be located precisely at the fracture site.
It has been known for some time that the provision of a direct current flowing from a cathode at or near a bone fracture to an anode taped to a patient's skin will stimulate bone growth at the fracture site with a direct current from 5 to 20 microamperes. U.S. Pat. No. 3,842,841 issued to Brighton, et al., on Oct. 22, 1974, entitled "Constant Current Power Pack for Bone Healing and Method of Use" discloses such a system. A publication by Zimmer.multidot.USA, Inc., Literature No. B-2360-1, revised in September, 1979 and available from Zimmer.multidot.USA, 727 N. Detroit Street, P. O. Box 708, Warsaw, Ind. 46580, provides a detailed discussion of direct current bone growth stimulation and an advantageous system to implement such stimulation.
Briefly, the disclosed system utilizes a plurality of cathodes equipped with a drill tip which can be inserted into the fracture site. In preferred embodiments, the drill tip first goes through bone tissue and then into the fracture site. A suitable power pack is connected to each of the four cathodes and, in the preferred embodiment, a 20 microampere current is supplied to each of the four cathodes. An anode pad is placed some distance from the cathode location and serves to complete the direct current circuit. Such a system has been found to be extremely beneficial in stimulating osteogenesis, especially in the case of an extended nonunion of a fracture (two years or more).
Obviously, when four separate cathodes are inserted into or towards the fracture site, these provide four possible sources of infection and/or local irritation. Additionally, there are four cathodes which must be maintained in the proper position in order to stimulate the osteogenesis at the desired location. If only a single cathode is used, the maximum applicable current is 20 microamperes and higher amounts of current result in tissue necrosis and localized burning in the event of high current densities.
It was reported by Brighton, et al., at the Annual Meeting of the Orthopaedic Research Society, in Dallas, Tex., Feb. 21, 1978 that the actual length of the cathode did not seem to affect the amount of bone formed when non-drill cathodes were inserted in the medullary canal of test animals. It was found that, where the current level of a conventional cathode was increased, there was no increase in new bone formed and rather extensive tissue necrosis occurred at higher current values. This was not true where a single cathode had a number of small bare spots along the length of the cathode. In this configuration, a single cathode would produce the same amount of bone at each bare spot as would the conventional cathode having just a single insulation-bare wire junction. Further studies showed that the bare area at the insulation-bare wire junction on a conventional cathode was a very small area and the length of the bare wire extending beyond this insulation-bare wire junction was irrelevant to bone formation. A protein precipitate forms over the remainder of the bare wire such that current density at the end of the cathode is independent of the length of the exposed wire extending beyond the insulation-bare wire junction.
An experimental cathode was tested as reported in the above-referenced Annual Meeting, which cathode was completely covered with an insulation material with the exception of eight bare spots in the insulation which exposed the cathode. This experimental electrode was useable only in comparison with the other test electrodes in the medullary canal which easily accepted the wire cathode. It was clear that such an electrode was not readily useable in the treatment of fractures either in animals or humans without further research and development. Because the medullary canal in the test animals is not considered a fracture model, no conclusions or suggestions could be drawn regarding fracture site osteogenesis from the Annual Meeting report. The problem of initially locating the cathode in the vicinity of the fracture site and maintaining its position therein is not addressed in the report and, of course, is critical in any clinical application of such a cathode.